1. Field of the Invention
The present invention relates generally to a process and apparatus for providing an inert atmosphere at a coal mining face during a mining operation to prevent the formation of an explosive mixture. More particularly, the present invention provides such an inert atmosphere by causing the volume of coal carried away from a mine face to be replaced by an inert gas by utilizing the coal flow as the impetus for the flow of inert gas to the mine face from the introduction site.
2. Description of the Prior Art
While not so limited, the present invention is particularly useful incident to the extraction of coal by highwall mining methods. Two such highwall coal mining methods known in the art are: first method using an auger type cutting and an auger conveying system and; more recently a second method using a continuous mining type cutting machine combined with a belt type conveying system. These highwall mining methods enable the recovered coal from a reserve when removal of the overburden by surface mining equipment becomes uneconomical. When augers are used, the auger enters the coal seam while rotating about an axis that is generally horizontal by a drive from a mine bench at the highwall. Coal is won by drilling a series of generally parallel holes that penetrate the mine seam up to distances of 400 or more feet. The auger device for cutting the coal seam are well known in the art. It is to be noted that augers and other coal cutting devices can be used to win coal wherein the rotational axis of the cutting device, when in the form of an auger, is orientated vertically or at an angle ranging between the vertical and horizontal. At very shallow depths little methane has been encountered with auger mining particularly when the penetration depth by the auger is of a shallow depth because methane is usually depleted by natural dissipation to the atmosphere when coal is near the ground surface. Pockets of methane may be encountered at depths as small as 35 feet. Conventional auger mining involves a penetration depth usually between 100 and 200 feet. However, penetration depths of 1000 and even as high as 1200 feet in to a highwall have been accomplished by the use of a continuous haulage system made up of a tandem arrangement of cascading conveyors that can convey the coal in an uninterrupted fashion from the coal face where coal is released by a continuous mining all per se well known in the art.
A severe problem of uncontrolled ignition, i.e., explosion, occurs without advance warning when pockets of methane are encountered during the coal releasing operation. The occurrence of quantities methane in excess of 2% of the volume in the interior of a highwall auger hole is unacceptable when there concurrently exists sufficient oxygen that will allow combustion. Moreover, an explosive environment may be created by a fuel source other than or in combination with methane such as coal dust. As is known in the art, coal dust attains an explosive condition when the dust mixture has 80 grams per cubic meter in atmospheric air under standard condition. As methane is added to the air - dust mixture, a liner relationship exists between these two components under explosive conditions. For example at 2.5% methane (i.e. 50% of the explosive limits methane is under) 40 grams of dust per cubic meter renders the mixture explosive.
Even when the necessary components and conditions exist for combustion to occur at the mine face, the difficulties to disarm the components and conditions by ventilating are compounded by a necessity of assuring the reliability of disarmament. When some ventilation to the coal cutter site at the front of the mining head is achieved, the ventilation medium may combine with methane to convert a methane rich but nonexplosive mixture to an explosive mixture. Also, ventilation is ineffective to prevent a dust explosion in such a mining configuration. An ignition source is often present in an auger type mining machine because the use of water or air to cool coal cutting bits is not practicable or reliable as a measure to always prevent the development of an ignition source.
The process of using inert gas to mine coal has been considered, for example, in Feasibility Study on Mining Coal in an Oxygen Free Atmosphere, Federal Water Quality Administration, Department of the Interior NTIS PB-197446, August 1970, 162 p. This report essentially dealt with air lock technology to place an entire conventional underground mine in an inert gas atmosphere and place the workers in "space suits". Use of inert gas to prevent explosions is per se, well known. The present invention eliminates the need for air lock technology while providing a method of supplying inert gas to an area which is otherwise inaccessible. Moreover, implementation of the inert gas to suppress combustion in underground mining is complicated and complex because of, inter ali, the underground environment. However, the underling discovering of the present invention is that the immediate vicinity of a coal face from which coal is being released can be inerted irrespective of whether the coal face is accessed from the surface or from an underground mine to prevent explosions at the mining site. Moreover, it has been discovered that the placement of inert gas can be effectively performed to not only assure movement of the inert gas to the cutter head but also in a manner that will allow monitoring of the hole atmosphere.